Seismic surveys image or map the subsurface of the earth by imparting acoustic energy into the ground and recording the reflected energy or “echoes” that return from the rock layers below. Each time the energy source is activated, it generates a seismic signal that travels into the earth, is partially reflected, and, upon its return, may be recorded at many locations on the surface as a function of travel time.
In the process of acquiring seismic data, a crew is typically deployed across several square miles of a survey area positioning cables and seismic receivers while seismic sources move from predetermined point to predetermined point to deliver vibrational seismic energy into the earth. The receivers record the reflected signals and the recordings are subsequently processed to develop images of geologic structures under the surface.
A land survey typically uses one of two energy sources to generate the down going seismic signal: either an explosive source or a vibrational source. Of particular interest for purposes of this disclosure is the use of seismic vibrator. A seismic vibrator generally takes the form of a truck or other vehicle that has a base plate that can be brought into contact with the earth. Conventionally, a reaction mass in association with a baseplate is driven by a system to produce vibratory motion, which travels downward into the earth via the base plate.
The vibroseis system was introduced by ConocoPhillips in the 1960s, and since that time, it has become the preferred source in those areas where it can operate. A vibrator is excited by a pilot signal that varies in frequency. In addition to the fundamental frequency, harmonics are generated by non-linearities in the vibrator mechanism and the earth. The nonlinearities include nonlinear coupling of the vibrator to the ground, the nonlinear effects in the vibrator itself, and inadequacy of the feedback system. Thus, an important issue of vibroseis data enhancement is the treatment or suppression of upper harmonics.
The vibrational source generates harmonics which, in certain circumstances, can have an energy approaching or even exceeding the fundamental signal, and which can crossfeed with signals from other sources, giving misleading results when the signals are processed to separate the signals from each source. In addition, the harmonics are a source of noise and can mask weak reflection signals from deeper layers.
The receivers that are used to detect the returning seismic energy for the land survey usually take the form of sensors like geophones or accelerometers. The returning reflected seismic energy is acquired from a continuous signal representing displacement, velocity or acceleration that may be represented as an amplitude variation as a function of time.
Typically, thousands of discrete seismic receivers are used to gather seismic data. The seismic receivers are generally laid out in lines that are substantially parallel and laterally spaced at equal distances and uniformly spaced down the line. In this configuration, uniform coverage of the subsurface is achieved. It is conventional that receiver spacing along the lines is closer than the spacing between the lines and that, therefore, the wavefield detected by the sensors is less well sampled in the lateral direction (perpendicular to the receiver lines) in most seismic surveys. Other survey designs are possible, but this layout is typical.
The receivers may also detect noise generated from one or more seismic sources that are not part of the seismic survey or from overlapping sweeps. These noises may distort the acquired seismic data by, for example, overlapping with a main reflected seismic wavefield that is the aim of the survey's measurement. The noises may also reach the receivers from directions that are significantly different with respect to the main reflected seismic wavefield. For instance, during slip-sweep acquisition, a second vibrator group starts its sweep before the end of the listen time of the first sweep. This overlapping approach is different from simultaneous shooting in that it does not require the vibrators to be ready at their locations at the same time. However, harmonic noise from adjacent sweeps can leak into each other, contaminating the records.
Due to the increasing reliance on seismic surveys for oil field development and the push for simultaneous sourcing of the surveys, much effort has been made to improve survey designs to either supress the noise or to separate the noise from the signal. Thus, there exists a need for methods that improve the quality of acquired seismic data by removing the effects of harmonics and other types of noise.